Color filter arrays formed of additive primary filters (blue, green, and red filters) laterally displaced in interlaid patterns have been used for imaging since near the turn of the century. Additive primary multicolor images were formed using a continuous, panchromatically sensitized silver halide emulsion layer exposed and viewed through an array of additive primary filters. Exposure through the filter array allows silver halide to be selectively developed either in exposed or unexposed areas. A multicolor image can be viewed by reflection or, more commonly, projection through the developed silver and color filter array.
Dufay U.S. Pat. No. 1,003,720 teaches forming an additive multicolor filter by alternately printing two thirds of a filter element with a greasy material to leave uncovered an array of areas. An additive primary dye is imbibed into the filter element in the uncovered areas. By repeating the sequence three times the entire filter area is covered by an interlaid pattern of additive primary filters. Rheinberg U.S. Pat. No. 1,191,034 attempts an essentially similar effect by using subtractive primary dyes (yellow, magenta, and cyan) which are allowed to diffuse laterally so that two substractive primaries are fused in each area to produce an additive primary color fitler array. In more recent investigations of imaging with additive multicolor filters placement of the additive primary filters in microcells to control their lateral definition is taught by Blazey et al U.S. Pat. No. 4,307,165, Whitmore U.S. Pat. No. 4,387,146, and Gilmour et al U.S. Pat. No. 4,411,973.
Color filter arrays comprised of interlaid patterns of additive primary filters have also been employed in connection with image sensors. Forming color filter arrays useful with semiconductor sensors has proven particularly challenging because of the small individual sensor areas, commonly less than 1.times.10.sup.-8 m.sup.2 in area, with areas of less than 1.times.10.sup.-10 m.sup.2 often being sought. Hartman U.S. Pat. No. 4,315,978 is considered representative. Filters of one additive primary color are formed by imagewise exposing a layer comprised of a combination of a mordant and a photopolymer. Following development to remove unexposed portions of the layer, the additive primary dye is imbibed. The process is then twice repeated to produce the second and third sets of additive primary filters.
One difficulty encountered in preparing the color filter arrays of Hartman is that combinations of dyes within a single filter element are required. In theory a blue dye transmits blue (400 to 500 nm) light while absorbing green and red (500 to 700 nm) light, a green dye transmits green (500 to 600 nm) light while absorbing blue and red (400 to 500 nm and 600 to 700 nm) light, and a red dye transmits red (600 to 700 nm) light and absorbs blue and green (400 to 600 nm) light. In practice rarely does any one dye absorb light efficiently throughout the two thirds of the visible spectrum in which absorption is desired. The result is that mixtures of additive primary dyes are usually required to approximate the theoretically desired absorption profiles. When different dyes are imbibed to form the same filter element, disadvantages are encountered because of the difficulty in predicting and reproducibly controlling the proportions of the different dyes. The result can be wide variations in absorption and transmission characteristics.
In addition to teaching the use of additive primary dyes, Hartman teaches that mixtures of subtractive primary (yellow, magenta, and cyan) dyes can be used to form additive primary filters. Using mixtures of subtractive primary dyes carries the same disadvantages as using dye mixtures in general and, in addition, because of the necessarily diverse absorption and transmission profiles of the individual subtractive primary dyes which must be mixed in a single filter element, the potential for variations in absorption and transmission characteristics is increased.
Aono U.S. Pat. No. 4,294,900 teaches producing additive primary filters by superimposing two silver halide emulsion layers containing subtractive primary dyes. Aono employs in the silver halide emulsion layers dyes of a specialized type called colored couplers of one subtractive primary color which on coupling with oxidized developing agent following imagewise exposure form dyes of another subtractive primary color. This approach does not permit the precise control of dye hue or location. It is well known that some unwanted silver halide grain development, referred to as fog or minimum density, always occurs. At the same time incomplete development of exposed silver halide is quite common. The result is that the filters of Aono contain unwanted mixtures of colored coupler dyes of one hue and coupling formed dyes of another hue. Further, the competing reactions often occur during coupling, leading to reaction product mixtures. The scattering of light during exposure and the migration of oxidized developing agent to effect coupling inherently results in only loosely controlled dye boundaries. Finally, using silver halide emulsions non-uniformities in dye patterns, referred to as granularity, is an art recognized concern.
Multicolor filter arrays have been constructed that employ combinations of filters other than interlaid sets of blue, green, and red filters. Sasano et al European Pat. No. 30,476 is illustrative of the contruction of a filter array consisting of interlaid sets of first and second subtractive primary (e.g., cyan and yellow) filters and additive primary (e.g., green) filters which result from laterally extending and superimposing portions of the the subtractive primary filters. This filter arrangement is specifically designed for sensors of a type that require the green signal to be subtracted from the cyan signal to produce an accurate blue signal and the the green signal to be subtracted from the yellow signal to produce an accurate red signal. These filters differ in transmission and therefore construction requirements from blue, green, and red color filter arrays.